In electrophotographic copying, an electrostatic latent image is developed on a primary image-forming member, such as a photoconductive surface, and developed with a thermoplastic toner powder to form a toner powder image. The toner image is thereafter transferred to a receiver, such as a sheet of paper, plastic or the like, and the toner image is subsequently fused to the receiver in a fusing section using heat, pressure, or both. A fuser section includes fuser members, which are typically rollers, although fuser belts and the like may also be used. Typically, a pressure roller and a fuser roller are used and at least one of the two rollers is usually heated. The essential function performed in the fusing section is the application of heat and pressure to the toner powder image on the receiver to fix the image to the receiver. In the discussion of this invention, the terms “pressure roller” and “fuser roller” will be used to refer to the two rollers used in the fuser section. It should be understood that the reference to rollers also includes a reference to fuser belts and similar equipment that provide similar heat and pressure treatment for latent images on receiver sheets. The term roller is also used to refer to any roller or belt or other similar equipment used in the fusing system.
The fusing step is commonly carried out by passing the toner image-bearing receiver between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form the nip, at least one of the rollers typically includes a compliant or conformable layer. Heat is transferred from at least one of the rollers to the toner in the fusing nip causing the toner to partially melt and attach to the receiver. In the case where the fuser member is a heated roller, a resilient and compliant pressure roller having a smooth surface is typically used. Where the fuser member is in the form of a belt, such as a flexible endless belt that passes around the heated roller, it typically has a smooth, hardened outer surface.
Most fuser sections, which are known as simplex fusers, attach toner to only one side of the receiver at a time. In such fusers it is common for a first of the two rollers to be driven rotatably by an external source. The second roller is then rotatably driven by frictional contact with the first roller. Similarly, heat is typically applied to only one of the rollers. The heat may be applied by the use of one or more heat rollers to heat the exterior of the heated fuser roller or the heat may be applied internally to the heated fuser roller.
Two basic types of heated rollers have been used. One type uses a conformable or compliant pressure roller to form a fusing nip against a hard, heated fuser roller. The other uses a compliant fuser roller to form the nip against a hard, heated and relatively non-conformable pressure roller. A fuser roller designed as compliant typically comprises a conformable layer having a thickness greater than about 2 millimeters (mm) and in some instances greater than about 25 mm. A fuser roller designated as hard comprises a rigid cylinder that may have a relatively thin polymeric or conformable elastomeric coating less than about 1.25 mm thick on its exterior. There are certain advantages associated with both compliant and noncompliant rollers. Typically, fuser rollers include a conformable layer that may be formed of any suitable material, such as for instance, polydimethylsiloxane elastomer.
Typically, fuser rollers include a hollow cylinder core, which is often metallic, with a roller cushion layer formed about the roller. Such cushion layers are commonly made of silicone rubbers or silicone polymers having a low surface energy such as polydimethylsiloxane, which minimizes the adherence of toner to the roller, especially the heated rollers. It is also known that cured polyfluorocarbon polymers and copolymers may be used to coat the cushion layer surface to further reduce the tendency of the toner to adhere to the roller and minimize contact of release oils with the cushion layer.
The cushion layer may include fillers comprising inorganic particles such as metals, metal oxides, metal hydroxides, metal salts, mixtures thereof and the like. These materials function to improve the thermal conductivity of the cushion layer. The use of such filler particles is disclosed in U.S. Pat. No. 6,393,249B1 issued May 21, 2002 to Muhammed Aslam et al. and assigned on its face to NexPress Solutions LLC. This patent is hereby incorporated in its entirety by reference.
It is also known that various fluoropolymers, such as thermoplastic fluorocarbon polymers and random copolymers are useful as coatings on such rollers. Some fluorocarbon thermoplastic polymers and thermoplastic random copolymers, including various materials, are disclosed in U.S. Pat. No. 6,355,352B1 issued Mar. 12, 2002 to Jiann H. Chen et al. and assigned on its face to NexPress Solutions LLC, U.S. Pat. No. 6,361,829 issued Mar. 26, 2002 to Jiann H. Chen et al.; U.S. Pat. No. 6,372,833B1 issued Apr. 16, 2002 to Jiann H. Chen et al. and assigned on its face to NexPress Solutions LLC; and U.S. Pat. No. 6,429,249B1 issued Aug. 6, 2002 to Jiann H. Chen et al. and assigned on its face to NexPress Solutions LLC. These patents are hereby incorporated in their entirety by reference.
While silicone rubbers and silicone polymers have been used widely as cushion layers, they have also in some instances been used as an exterior layer. Fluoroelastomers and rubbers, such as rubbers made of ethylene propylene diene monomers and the like have also been used as cushion materials. Unfortunately in many fusing processes the exterior of the fuser roller in direct contact with the toner, especially a heated fuser roller, is coated with a release oil during fusing. Such release oils are generally detrimental to silicone rubbers and silicone polymers. Polyfluorocarbon polymers and random copolymers coated over the outside of the cushion have been found to be resistant to such oils and provide a low energy surface which has improved release properties from the toner and the receiver and are not adversely effected by the commonly used oils.
Notwithstanding the availability of such polymers having improved release functions, it is desirable to further improve the release properties of the receivers and toners from the fuser roller quickly after the receiver sheets leave the nip. Continued efforts have been directed to the development of improved exterior coatings for fuser rollers that will provide for immediate release of the receiver sheet from the rollers, especially the heated fuser roller after the fusing operation has been concluded. Continued efforts have been directed to the development of low damping fluorocarbon thermoplastic random copolymers for use as exterior coatings on the fuser rollers.